The effectiveness of an emission control system in a lean-burn internal combustion engine can be improved by increasing the temperature of the effluent flowing into the emission control system. For example, raising the temperature of the exhaust gas can improve the absorption efficiency of lean NOx traps (LNT), which may reduce tailpipe emissions from the vehicle. The temperature of the effluent can be raised by incorporating an exhaust gas recirculation (EGR) system, which recycles a portion of the engine effluent to the engine intake passage. For example, combustion systems utilizing higher EGR flow rates may exhibit lower combustion temperatures and higher exhaust temperatures.
In turbocharged diesel engines, throttling the fresh air intake while introducing heated, pressurized recirculated exhaust to the engine intake manifold may be used to increase exhaust temperature. However, such operation may also cause the compressor to stall or surge. The low mass flow rate through the compressor, in combination with the high pressure ratio of the compressor, can create a flow instability inside the compressor. The resulting “compressor surge” can cause damage to the turbocharger, and the fluctuations in air flow out of the compressor can affect the air to fuel ratio, which can ultimately affect the combustion stability of the engine.
The above issue may be addressed by, in one example, a method of operating a compressor recirculation valve on an engine having a throttle coupled to the turbocharger. The engine may include a high pressure exhaust gas recirculation (EGR) system for recirculating cooled and un-cooled exhaust gas from upstream of a turbine of the turbocharger in an exhaust to downstream of a compressor of the turbocharger in an intake. The compressor may have a recirculation passage with a recirculation valve coupled therein. The method may comprise during a first engine airflow condition:
delivering at least some high pressure un-cooled EGR to the intake, downstream of the compressor, operating the throttle at a first throttle amount, and operating the compressor bypass valve at a first bypass amount to increase exhaust gas temperature; and
during a second engine airflow condition higher than the first engine airflow:
delivering at least some high pressure cooled EGR to the intake, downstream of the compressor, operating the throttle at a second throttle amount more open than the first amount, and operating the compressor bypass valve at a second bypass amount more closed than the first amount.
For example, during the first condition, by recirculating air through the compressor at its inlet the compressor still pumps air even though the recirculated air does not go through engine. Such operation therefore does not further increase intake manifold pressure, thereby reducing a likelihood of surge. Further, by pumping extra air, the compressor flow is increased, also decreasing a likelihood of surge. Further still, recirculating air through the compressor also helps maintain loading on turbine thus maintaining sufficient turbine inlet pressure to pump the high pressure EGR to the intake and thereby enable increased exhaust gas temperature for maintaining activation of an exhaust catalyst.
As another example, by adjusting intake throttling as described during the first condition, it may be possible to provide sufficient pressure differential so that increased EGR and/or near stoichiometric combustion can be achieved to also increase exhaust temperature.
As still another example, by adjusting operation as described during the second condition, high engine efficiency and engine output can be achieved when desired. In this way, overall engine operation may be improved during both higher and lower airflow operation.
Thus, adjustable recirculation of air into the compressor inlet can be used to address surge issues that primarily occur at lower airflows where higher un-cooled EGR may be used to advantage.